1. Field of the Invention
The present invention relates to an electric sewing machine and, more particularly, to a controlling apparatus of an electric sewing machine suitable for use in an industrial sewing machine having an automatic needle stopping device.
2. Description of the Prior Art
In general, industrial sewing machines are provided with automatic devices, such as trimmers for cutting threads, wipers for wiping severed threads etc, which are to be controlled in accordance with a complicated sequence.
At the same time, it is highly desirable that the sequence can readily be changed or modified and/or optionally provided with additional functions.
Further, in order to avoid accidents causing injury to the human body due to the breakage of the needle, the sequence control of the sewing machine has to be highly reliable, and should be free from malfunctions attributable to noises and other reasons.
A typical example of a controlling apparatus of the kind described is disclosed in the specification of U.S. Pat. No. 3,761,790.
Conventional apparatus of this kind usually incorporates a group of switches, such as a switch LSW adapted to be turned on when a foot pedal is depressed forward, a yarn cutting switch TSW adapted to be turned on when the foot switch is depressed backward, an upper needle stop switch USW adapted to be operated by the operator's knee, and so forth.
In order to start the sewing machine, the foot pedal is depressed forwardly, so as to turn the switch LSW on, thereby to actuate a speed controlling circuit.
The speed controlling circuit in turn provides an operation signal, through a clutch driving circuit, to the winding of the clutch to energize the same. At the same time, another operation signal is delivered to a braking winding, through a brake driving circuit, thereby to energize the same as required.
An automatic controlling circuit is provided for distributing operation signals to the clutch winding and the brake winding, for comparing the output signal from a tachogenerator adapted to detect the rotational speed of the driving shaft of the machine with a voltage signal representative of a preselected speed of the machine, thereby to operate the machine always at the preselected speed.
Also, a control circuit is provided for controlling the stopping position of the needle. To this end, the circuit is adapted to detect two positions, i.e. an upper position in which the needle assumes the uppermost position in its stroke and a lower position in which the needle assumes the lowermost position in its stroke, and, in cooperation with the state of the switches of the aforementioned switch group, to select the needle stopping position. More specifically, the lower position is selected when the switch LSW has been turned on while the upper needle stopping switch USW has been turned off, whereas the upper position is selected whenever the upper needle stopping switch USW has been turned on, even if the switch LSW has been turned on. At the same time, the upper position is selected whenever the thread cutting switch has been turned on.
Assuming here that the lower position has been selected, due to turning on and off of the switches LSW and USW, respectively, the selected position detecting signal is made valid by a positioning speed detecting signal. Then, an operation signal for braking of a certain duration is generated by, for example, a monostable multi-vibrator for stopping, when the lower position is detected, so as to apply a voltage to the brake winding through the brake driving circuit, thereby to stop the machine shaft at the lower position.
Assuming that the upper stopping switch USW is turned on during the deceleration and stopping operation of the machine, the upper position is selected as stated above to allow the machine shaft to stop with the needle at the upper position.
Once the foot pedal has been depressed backward, the thread cutting switch TSW is turned on, so as to put the speed control circuit into operation and, at the same time, to apply a voltage to the trimmer coil through the trimmer driving circuit, thereby to start the thread cutting operation. In this case, the position selecting circuit selects the upper position, so that the brake is energized for a certain period of time. At the same time, since the thread cutting switch is turned on, a driving circuit for a wiper for wiping the severed thread is actuated to apply a voltage to a wiper coil for a certain period of time, thereby to effect the wiping of the thread.
As has been explained, the controlling circuit is normally equipped with various devices such as a group of input switches including the switches LSW, TSW and so on, position detecting means and so forth.
The long wires through which these devices are connected to the controlling circuit are likely to invite external noises, often resulting in erroneous operation of the controlling circuit which may incur a serious accident, such as breakage of the needle.
In addition, when it is required to alter and modify the sequence or to add a new function to the controlling circuit, troublesome modifications are required for rearranging the wirings and circuits, since these wirings and circuits are usually fixed.
Further, an impractically large number of parts in the circuit, as well as highly complicated wirings inevitably lead to an increased cost, deterioration of reliability and malfunctions attributable to the mutual interference between the circuits.